How Much CFM Your Ducts Need, Room by Room
Estimate your HVAC system's total airflow from its tonnage (about 400 CFM per ton) and one room's share of that airflow by floor area.
How it works
Ductwork is sized around airflow, measured in CFM — cubic feet of air per minute. The starting rule for a residential system is about 400 CFM per ton of cooling capacity, so a 3-ton system moves roughly 1,200 CFM total. Humid climates often design a little lower, near 350 CFM/ton, to give the coil more time to wring out moisture; dry climates run higher, near 450.
Once you know total airflow, a first-pass way to split it among rooms is by floor area: a room gets the share of total CFM equal to its share of the conditioned square footage. A 150 square-foot bedroom in a 1,500 square-foot home is 10% of the area, so it gets about 10% of 1,200 CFM, or 120 CFM. That sets a target for the supply register and branch duct feeding that room.
This area split is a planning estimate, not a duct design. Real rooms differ in heat load — a west-facing room with big windows needs more air than a shaded interior room of the same size — so a proper job uses a room-by-room Manual J load calculation feeding a Manual D duct design, then balancing dampers to fine-tune. Use these numbers to sanity-check registers and spot rooms that are starved for air.
total airflow = 3 × 400 = 1,200 CFM (2,039 m³/h)
room airflow = 1,200 × (150 ÷ 1,500) = 120 CFM